Modern electronic devices such as a notebook computer comprise a variety of memories to store information. Memory circuits include two major categories. One is volatile memories; the other is non-volatile memories. Volatile memories include random access memory (RAM), which can be further divided into two sub-categories, static random access memory (SRAM) and dynamic random access memory (DRAM). Both SRAM and DRAM are volatile because they will lose the information they store when they are not powered. On the other hand, non-volatile memories can keep data stored on them. Non-volatile memories include a variety of sub-categories, such as read-only-memory (ROM), electrically erasable programmable read-only memory (EEPROM) and flash memory.
Flash memory is a non-volatile device that can be electrically erased and reprogrammed. A typical flash memory comprises a memory array having a large number of flash memory cells arranged in rows, columns, and blocks. One of the most commonly known flash memories is the one-transistor flash memory. The memory cell of the one-transistor flash memory is fabricated as a field-effect transistor having two gates, namely a control gate and a floating gate. The floating gate is capable of holding charges and is separated from source and drain regions.
Each of the memory cells can be electrically charged by injecting hot electrons across an oxide layer (tunneling layer) onto the floating gate. The charges can be removed from the floating gate by tunneling the electrons to the substrate through the tunneling layer during an erase operation. Thus the data in a memory cell is determined by the presence or absence of charges in the floating gate.
As technologies evolve, vertical transistors are being researched recently. In a vertical transistor, a vertical nanowire formed over a substrate comprises a source, a channel and a drain of the vertical transistor. A gate dielectric and a gate electrode are formed to encircle the vertical nanowire. The encircled portion of the vertical nanowire forms the channel of a vertical transistor. The vertical transistor has a gate-all-around structure since the channel is surrounded by the gate electrode. Such a gate-all-around structure helps to minimize the short-channel effects of the vertical transistor.